Conventionally, a pattern of metal deposit or impurity doping on a substrate is formed as follows:
a. coat the substrate with a photoresist layer;
b. prepare a photomask (or a reticle) made of a transparent plate (such as of glass or quartz crystal) with an opaque pattern described on it;
c. cast light through the photomask on the photoresist layer;
d. process the photoresist layer with a proper solvent, whereby only the exposed part (or the unexposed part) of the layer is washed out to reveal an aperture in the photoresist layer having the same pattern on the substrate;
e. metal is deposited or impurity dopant is cast on the patterned photoresist-substrate; and
f. take away the remaining photoresist layer to obtain the desired pattern of metal deposit or impurity doping.
In order to increase the density of an IC chip the number of parts included in one chip) and to improve basic characteristics of an IC chip as a microwave device, every size of the pattern elements (typically, width of a line) should be minimized. But the width of a line is limited by the resolution limit of the photoresist material: for example, when ultraviolet light having wavelength of 365 nm (i-line UV) is used, the photoresist pattern obtained in step d cannot have a line narrower than the resolution limit of the photoresist material (about 0.4 .mu.m).
The photolithographic method of the present invention is based on the `image reversal process`, which is detailed in "AZ (trade name) 5200 Resists for Positive- and Negative Patterning" by Michael Bolsen, Hoechst Japan Limited, Electronic Materials Division (Japanese parallel is disclosed in pp. 43-48 of June, 1989 issue of a Japanese magazine "Electronic Material").